Add user satisfaction survey

This adds the user satisfaction survey back to GOV.UK. A change from the last time this was run is that it's now appearing across the entirety of GOV.UK and not just mainstream content pages.

A caveat to this is that it's not to appear on the following transaction done pages:

- gov.uk/done/*
- gov.uk/transaction-finished
- gov.uk/driving-transaction-finished

#### Testing (visuals) locally

There are two ways you can test this and they depend on whether you're using a browser with or without a console.

- For newer browsers run the following function in the console to view the survey: `GOVUK.userSatisfaction.showSurveyBar();`
- For older browsers you can change **line 62** of the file `app/assets/javascripts/user-satisfaction-survey.js` to `GOVUK.userSatisfaction.showSurveyBar();` and load a given URL in the browser